The present invention relates to a low-chromium ferritic heat-resistant steel which exhibits a high creep strength at a temperature of 550.degree. C. or higher and excellent low temperature toughness at room temperature or lower. The steels of the present invention are particularly useful in making heat exchange pipes, piping, heat resistant valves, and connecting joints which are produced through casting and forging, four example, in the boiler-making industry, the chemical industry, and the atomic power industry.
Conventionally, in the manufacture of heat-resistant, pressure-resistant members which are mainly used in such industrial fields as mentioned above, austenitic stainless steels, high-Cr ferritic steels (Cr content of 9-12%), low-Cr Mo-containing ferritic steels (Cr content of 3.5% or smaller), or carbon steels have been used. Among these, suitable steel is selected in view of the service temperature and pressure as well as the circumstances under which the member is to be used. Economy is also important. For example, low-Cr-Mo system ferritic steels with a Cr content of 3.5% are characterized in that they are highly resistant to oxidation and corrosion and exhibit an excellent high temperature strength in comparison with carbon steels which do not contain Cr. In comparison with austenitic stainless steels, this Cr-containing steel is also inexpensive, it is free from stress corrosion cracking, and it has a small thermal linear expansion coefficient. In addition, in comparison with high-Cr ferritic steels, this low Cr-containing steel is less expensive and is superior with respect to toughness, thermal conductivity, and weldability.
Typical examples of low Cr-containing steel are JIS SFBA 24 (21/4Cr-1Mo Steels), STBA 22, and STBA 20, which are collectively referred to as Cr-Mo steels.
Precipitation-hardenable elements, such as V, Nb, Ti, Ta, and B may be added to Cr-Mo steels. See Japanese.Unexamined Laid-Open Patent Specification No. 57-131349/1982, No. 57-131350/1982, No. 62-54062/1987, No. 63-62848/1988, No. 6468451/1989, No. 63-18038/1988, No. 3-64428/1991, and No. 3-87332/1991, and Japanese Patent Publication No. 1-29853/1989.
For use in making turbines, 1Cr-1Mo-0.25V steels are well . known, and for use in constructing fast breeder reactors, 21/4Cr-1Mo-Nb steels are well known.
However, the above-mentioned low Cr-steels are inferior to high Cr ferritic steels and austenitic stainless steels with respect to their resistance to high temperature oxidation and corrosion, and have a much lower strength at high temperatures. Thus, they have troubles if used at a temperature higher than 550.degree. C.
Japanese Unexamined Laid-Open Patent Specifications NO. 2-217438/1990 and No. 2-217439/1990 propose low Cr heat-resistant steels which exhibit improved resistance to high temperature oxidation and corrosion, have excellent high temperature strength, and can be used in place of high-Cr ferritic steels and austenitic stainless steels.
Since the resistance of a steel to oxidation and to corrosion at high temperatures mainly depends on the Cr content of the steel, it is effective to increase the Cr content order to improve such properties. However, the larger the Cr content the lower the thermal conductivity, toughness, weldability, and economy. The invention disclosed in the above-mentioned Japanese Unexamined Laid-Open Patent Specification No. 2-217439/1990 is directed to steels having an oxidation resistance improved by the addition of Cu without increasing the Cr content.
On the other hand, the high temperature strength of a material is critical when the material is used to form a high pressure-resistant member. It is desirable that the high temperature strength always be great regardless of service temperatures. This is because in heat and pressure-resistant steel pipes, such as those used in boilers and in tubes or containers for the chemical and atomic power industries, the wall thickness of a pipe or tube or container is determined by its high temperature strength.
In addition, toughness is critical for pressure-resistant piping, especially when welding is employed in connecting piping. This is because welds are sometimes more brittle than the base material due to inhomogeneities in structure. If the toughness of a material is substantially degraded, failure during pressure testing and fracture during construction or repair of the piping or structure might occur, resulting in less reliability of the structure.
Thus, the following advantages can be obtained when the high temperature strength as well as toughness of low Cr ferritic steels have been improved substantially:
1) In rather mild corrosive conditions at high temperatures, less expensive low-Cr ferritic steels can be used instead of austenitic stainless steels or high-Cr ferritic steels which have conventionally been used in order to ensure high temperature strength. PA1 2) It will be possible to further reduce the wall thickness, resulting in an improvement in thermal conductivity. Thus, the thermal efficiency of equipment can be improved and thermal fatigue of the equipment, which occurs when the equipment is started and shut down, can also be relieved. PA1 3) It is also possible to make the equipment compact and to lower the manufacturing costs due to the lightening of structural elements. PA1 C : 0.03-0.12%, Si: 0.70% or less, Mn: 0.10-1.50%, PA1 Ni: 2.0% or less, Cr: 1.50-3.50%, W: 1.0-3.0%, PA1 V: 0.10-0.35%, Nb: 0.01-0.10%, PA1 B: 0.00010-0.020%, N: 0.005% or less, PA1 Al: 0.005% or less, PA1 Ti: not less than 0.001 but less than 0.05%, PA1 Cu: 0.10-2.50%, PA1 optionally at least one element from the following (i)-(ii):
Thus, it is apparent that low-Cr ferritic steels with high strength can result in many practical advantages. However, conventional low-Cr steels with high strength have poor toughness.
For example, Cr-Mo steels, such as JIS STBA 22 and JIS STBA 24, which utilize solution hardening of Mo and precipitation hardening of fine carbides of Cr, Fe, and Mo do not exhibit a higher level of high temperature strength, since the contribution of solution hardening of Mo to an increase in high temperature strength is small and the precipitation hardening caused by the carbides is not so great as expected because of a rapid coarsening of the carbides. In order to increase high temperature strength, therefore, it is advisable to increase the Mo content. However, upon increasing the Mo content, it is inevitable that toughness, formability, and weldability are degraded greatly.
On the other hand, the addition of such precipitation hardening elements as V, Nb, Ti, and B is effective to improve the strength of steel. However, in this case, too, the steel in which carbides of these elements are precipitated in a ferritic matrix exhibits a marked degradation in toughness. Weldability is also deteriorated greatly.